Communications satellite


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Communications satellites play a major role in telephone transmission,television and radio program distribution,computer communications,maritime navigation and military command and control.
Satellite communications combines such diverse topics as Radio wave propagation ,antennas, orbital mechanics , modulation , detection , coding and radio electronics.

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Communications satellite

  2. 2. Absract:Communications satellites play a major role in telephone transmission,televisionand radio program distribution,computer communications,maritime navigationand military command and control. Satellite communications combines such diverse topics as Radio wavepropagation ,antennas, orbital mechanics , modulation , detection , coding andradio electronics.For fixed services, communications satellites provide a technologycomplementary to that of fiber optic submarine communication cables. They arealso used for mobile applications such as communications to ships and planes, forwhich application of other technologies, such as cable, are impractical orimpossible.
  3. 3. Contents: Introduction 1 History o 1.1 Early missions o 1.2 Geostationary orbits o 1.3 Low-Earth-orbiting satellites o 1.4 Molniya satellites 2 Applications o 2.1 Telephony o 2.2 Television and radio o 2.3 Mobile satellite technologies o 2.4 Amateur radio o 2.5 Satellite broadband 3 conclusion
  4. 4. Introduction: U.S. military MILSTAR communications satelliteA communications satellite (sometimesabbreviated to comsat) is an artificial.Modern communications satellites use Historygeosynchronous orbits, Molniya orbitsor low polar Earth orbits.For fixed Early missionsservices, communications satellites The first satellite equipped with on-provide a technology complementary to board radio-transmitters was Sovietthat of fiber optic submarine Sputnik 1, launched in 1957. The firstcommunication cables. They are also American satellite to relayused for mobile applications such as communications was Project SCORE incommunications to ships and planes, for 1958, which used a tape recorder to storewhich application of other technologies, and forward voice messages. It was usedsuch as cable, are impractical or to send a Christmas greeting to the worldimpossible. from President Eisenhower. NASA launched an Echo satellite in 1960; the 100-foot aluminized PET film balloon served as a passive reflector for radio communications. Courier 1B, (built by
  5. 5. Philco) also launched in 1960, was the satellite revolves around the earth at aworld’s first active repeater satellite. constant speed once per day over the equator.Telstar was the first active, direct relaycommunications satellite. Belonging to The geostationary orbit is useful forAT&T as part of a multi-national communications applications becauseagreement between AT&T, Bell ground based antennas, which must beTelephone Laboratories, NASA, the directed toward the satellite, can operateBritish General Post Office, and the effectively without the need forFrench National PTT (Post Office) to expensive equipment to track thedevelop satellite communication, it was satellite’s motion. Especially forlaunched by NASA from Cape applications that require a large numberCanaveral on July 10, 1962, the first of ground antennas (such as direct TVprivately sponsored space launch. Telstar distribution), the savings in groundwas placed in an elliptical orbit equipment can more than justify the(completed once every 2 hours and extra cost and onboard complexity of37 minutes), rotating at a 45° angle lifting a satellite into the relatively highabove the equator. geostationary orbit.An immediate antecedent of the The first geostationary communicationsgeostationary satellites was Hughes’ satellite was Anik 1, a Canadian satelliteSyncom 2, launched on July 26, 1963. launched in 1972. The United States launched their own geostationarySyncom revolved around the earth once communication satellites afterward, withper day at constant speed, but because it Western Union launching their Westar 1still had north-south motion, special satellite in 1974, and RCA Americomequipment was needed to track it. (later GE Americom, now SES Americom) launching Satcom 1 in 1975.Geostationary orbits: It was Satcom 1 that was instrumental inA satellite in a geostationary orbit helping early cable TV channels such asappears to be in a fixed position to an WTBS (now TBS Superstation), HBO,earth-based observer. A geostationary
  6. 6. CBN (now ABC Family), and The Low earth orbiting satellites are lessWeather Channel become successful, expensive to position in space thanbecause these channels distributed their geostationary satellites and, because ofprogramming to all of the local cable TV their closer proximity to the ground,headends using the satellite. require lower signal strength (Recall thatAdditionally, it was the first satellite signal strength falls off as the square ofused by broadcast TV networks in the the distance from the source, so theUnited States, like ABC, NBC, and effect is dramatic). So there is a trade offCBS, to distribute their programming to between the number of satellites andall of their local affiliate stations. Satcom their cost. In addition, there are1 was so widely used because it had important differences in the onboard andtwice the communications capacity of ground equipment needed to support theWestar 1 (24 transponders as opposed to two types of missions.Westar 1’s 12), resulting in lower A group of satellites working in concerttransponder usage costs. thus is known as a satellite constellation.Low-Earth-orbiting satellites: Two such constellations which were intended for provision for hand heldA low Earth orbit typically is a circular telephony, primarily to remote areas,orbit about 150 kilometres above the were the Iridium and Globalstar. Theearth’s surface and, correspondingly, a Iridium system has 66 satellites. Anotherperiod (time to revolve around the earth) LEO satellite constellation, with backingof about 90 minutes. Because of their from Microsoft entrepreneur Paul Allen,low altitude, these satellites are only was to have as many as 720 satellites.visible from within a radius of roughly1000 kilometres from the sub-satellite It is also possible to offer discontinuouspoint. In addition, satellites in low earth coverage using a low Earth orbit satelliteorbit change their position relative to the capable of storing data received whileground position quickly. So even for passing over one part of Earth andlocal applications, a large number of transmitting it later while passing oversatellites are needed if the mission another part. This will be the case withrequires uninterrupted connectivity.
  7. 7. the CASCADE system of Canada’s during the northern portion of the orbit.CASSIOPE communications satellite. (Elevation is the extent of the satellite’s position above the horizon. Thus aMolniya satellites: satellite at the horizon has zero elevation and a satellite directly overhead hasAs mentioned, geostationary satellites elevation of 90 degrees).are constrained to operate above theequator. As a consequence, they are not Furthermore, the Molniya orbit is soalways suitable for providing services at designed that the satellite spends thehigh latitudes: for at high latitudes a great majority of its time over the fargeostationary satellite may appear low northern latitudes, during which itson (or even below) the horizon, affecting ground footprint moves only slightly. Itsconnectivity and causing multipathing period is one half day, so that the(interference caused by signals reflecting satellite is available for operation overoff the ground into the ground antenna). the targeted region for eight hours everyThe first satellite of Molniya series was second revolution. In this way alaunched on April 23, 1965 and was constellation of three Molniya satellitesused for experimental transmission of (plus in-orbit spares) can provideTV signal from Moscow uplink station uninterrupted downlink stations, located in Siberiaand Russian Far East, in Norilsk, Molniya satellites are typically used forKhabarovsk, Magadan and Vladivostok. telephony and TV services over Russia.In November of 1967 Soviet engineers Another application is to use them forcreated a unique system of national TV mobile radio systems (even at lowernetwork of satellite television, called latitudes) since cars travelling throughOrbita, that was based on Molniya urban areas need access to satellites atsatellites. high elevation in order to secure good connectivity, e.g. in the presence of tallMolniya orbits can be an appealing buildings.alternative in such cases. The Molniyaorbit is highly inclined, guaranteeinggood elevation over selected positions
  8. 8. APPLICATIONS:TELEPHONY: Television and radio There are two satellite types used for North American television and radio:A BSS 601 model, owned by SES • Direct Broadcast Satellite (DBS),Astra, used for DTH television andbroadcasting in Europe • Fixed Service Satellite (FSS).The first and still, arguably, most A direct broadcast satellite is aimportant application for communication communications satellite that transmitssatellites is in international telephony. to small DBS satellite dishes (usually 18Fixed-point telephones relay calls to an to 24 inches in diameter). Directearth station, where they are then broadcast satellites generally operate intransmitted to a geostationary satellite. the upper portion of the Ku band. DBSAn analogous path is then followed on technology is used for DTH-orientedthe downlink. In contrast, mobile (Direct-To-Home) satellite TV services,telephones (to and from ships and such as DirecTV, DISH Network , andairplanes) must be directly connected to Sky Angel in the United States,equipment to uplink the signal to the ExpressVu in Canada, and Sky Digital insatellite, as well as being able to ensure the UK.satellite pointing in the presence of Fixed Service Satellites use the C band,disturbances, such as waves onboard a and the lower portions of the Ku bands.ship. They are normally used for broadcast
  9. 9. feeds to and from television networks FSS satellites differ from DBS satellitesand local affiliate stations (such as in that they have a lower RF powerprogram feeds for network and output than the latter, requiring a muchsyndicated programming, live shots, and larger dish for reception (3 to 8 feet inbackhauls), as well as being used for diameter for Ku band, and 12 feet on updistance learning by schools and for C band), as well as using linearuniversities, business television (BTV), polarization for each of the transpondersvideo-conferencing, and general RF input and output (as opposed tocommercial telecommunications. FSS circular polarization used by DBSsatellites are also used to distribute satellites). FSS satellite technology wasnational cable channels to cable TV also originally used for DTH satellite TVheadends. from the late 1970s to the early 1990s in the United States in the form of TVRO (TeleVision Receive Only) receivers and dishes (also known as big-dish, or more pejoratively known as "BUD" or "Big ugly dish" systems). It was also used in its Ku band form for the now-defunct Primestar satellite TV service. This all changed when the first American DBS provider, DirecTV, was established in 1994, stealing the limelight from FSS satellite technology for DTH programming (due to DirecTVs smaller 18-inch diameter dishes and lower equipment cost). However, FSS satellites on the C and Ku bands still are used by cable and satellite channels such as CNN, The Weather Channel, HBO, Starz, and others, for
  10. 10. distribution to cable TV head ends, and costs quite a bit more than a DBS systemto the DBS providers themselves such as (about US$1500–2000, includingDirecTV and DISH Network who then installation). But most older used TVROre-distribute these channels over their systems can be had almost for free, dueown DBS systems. to most people converting over to DBS systems over the years. Unlike DBS,The fact that these channels still exist on big-dish TVRO satellite TV alsoFSS satellites (more so for reception and provides a plethora of unscrambled andre-distribution by cable TV and DBS unencrypted channels such as Classicsystems, instead of for DTH viewers) Arts Showcase, and feeds of syndicatedmakes TVRO systems for DTH viewing TV shows for reception by local TVa still-viable option for satellite TV, stations.Free-to-air satellite TV channelsoften being a much-cheaper alternative are also usually distributed on FSSto DBS, as far as monthly subscription satellites in the Ku band. The Intelsatfees are concerned. TVRO-oriented Americas 5, Galaxy 10R and AMC 3programming packages sold by satellites over North America provide acompanies such as National quite large amount of FTA channels onProgramming Services,, their Ku band transponders.Theand Skyvision, are often quite a bit American Dish Network DBS servicecheaper than their DBS equivalents. has also recently utilized FSS technologyMotorola still makes digital 4DTV as well for their programming packagesreceivers for DTH TVRO use, and requiring their SuperDish antenna, dueanalog TVRO receivers are still to Dish Network needing more capacityavailable. to carry local television stations per the FCCs "must-carry" regulations, and forHowever, the hardware for a brand-new more bandwidth to carry HDTVTVRO system (dish and receiver, along channels.Satellites for communicationwith a VideoCipher or DigiCipher have now been launched that havedescrambler, or an integrated transponders in the Ka band, such asreceiver/decoder (IRD) like a 4DTV DirecTVs SPACEWAY-1 satellite. Thesystem, instead of a separate receiver definitions of FSS and DBS satellitesand descrambler/decoder) nowadays
  11. 11. outside of North America, especially in Amateur radio operators have access toEurope, are a bit more ambiguous. Most the OSCAR satellites that have beensatellites used for direct-to-home designed specifically to carry amateurtelevision in Europe have the same high radio traffic. Most such satellites operatepower output as DBS-class satellites as spaceborne repeaters, and areMobile satellite technologies: generally accessed by amateurs equipped with UHF or VHF radio equipment andInitially available for broadcast to highly directional antennas such asstationary TV receivers, by 2004 popular Yagis or dish antennas. Due to themobile direct broadcast applications limitations of ground-based amateurmade their appearance with that arrival equipment, most amateur satellites areof two satellite radio systems in the launched into fairly low Earth orbits, andUnited States: Sirius and XM Satellite are designed to deal with only a limitedRadio Holdings. Some manufacturers number of brief contacts at any givenhave also introduced special antennas for time. Some satellites also provide data-mobile reception of DBS television. forwarding services using the AX.25 orUsing GPS technology as a reference, similar protocols.these antennas automatically re-aim tothe satellite no matter where or how the Satellite broadband:vehicle (that the antenna is mounted on) In recent years, satellite communicationis situated. These mobile satellite technology has been used as a means toantennas are popular with some connect to the Internet via broadbandrecreational vehicle owners. Such mobile data connections. This can be very usefulDBS antennas are also used by JetBlue for users who are located in very remoteAirways for DirecTV (supplied by areas, and cannot access a wirelineLiveTV, a subsidiary of JetBlue), which broadband or dialup connection.passengers can view on-board on LCDscreens mounted in the seats. Conclusion:Amateur radio: Communicatios Satellites are widely used to distribute Television
  12. 12. programming to terrestrial Tv stations. COMSTAR satelliite systems ,Marisat is a U.S. system that serves the communications satelliteU.S. Navy and merchant marine , corporation,Washington,D.C.Comsat general is the system manager  Joseph N.Pelton,”INTELSAT:and majority stock holder. making the Future happen,” Space communication and broadReferences: casting.Jose l.Alegrettt,”U.S Role ininternationalSatelliteCommuni  COMSAT Guide to the cations II:Leadership Continues”, INTELSAT,MARISAT and Satellite Communications.